Add a filter to allow displaying threads in reverse order

[johnbillion]

In reponse to a support forum thread here: http://bbpress.org/forums/topic/reverse-the-display-of-the-messages

The get_thread() function allows a third parameter which, when set, reverses the order of the thread. Let's put a filter on this parameter inside the function which will allow a plugin to reverse the order of the thread.

Patch.

At it's most basic, a plugin can reverse the thread order like this:

add_filter( 'reverse_thread', create_function( '$a', 'return true;' ) );

In most cases, a complete plugin will contain conditionals to determine when to reverse the thread order (eg. based on a user setting or a URL parameter).

Filtering on "get_thread_order_by" will allow this. No need for the extra filter.